Hey! With those negative calorie diets, I think celery is really one of the only foods that ACTUALLY has negative calories. Most fruits and vegetables still have calories, since they're high in natural sugars, but they give you more of a boost in energy than other foods. However, the boost is only short term, so make sure that if you do decide to do this, you take protein supplements and calcium supplements (or just drink milk, because you need a little tiny bit of HEALTHY fat in your diet too), and then you should be good to go! You'd probably want to talk to a registered nutritionist or dietician first though, just to make sure that I've covered everything you need